I think I got the Vapor model 75-700:
http://www.trailtech.net/75-700.html
Most any model will work for just the tach function. Just be sure to specify that you need the tach wire with the 600k ohm series resistor in it, or you'll have to add that on your own.
The brass temperature sensor from the 75-700 unscrews from the black plastic pipe it comes in and fits inside the new-gen Ninja 250's thermostat housing (replacing the stock one).
I never did find a good way to mount the speed sensor on the fork, but was never really motivated to do that for a race bike.
The Vapor will need to be set for 1 pulse per revolution for the Ninja 250.